Add headers to table/tree widget

[benoitkugler]

Is your feature request related to a problem? Please describe:

I would like to title the columns of a table and to (externally) sort the rows when the header is clicked.

Is it possible to construct a solution with the existing API?

I think we can build a pseudo header with a second table, but it's not very clean. Especially, the possible horizontal scrolling need to synchronized.

Describe the solution you'd like to see:

The possibility to add headers (possibly both horizontal and vertical) to the table widget.
Ideally, horizontal headers could also be added to the tree widget.

They should provide a way to handle tap events, so that we can then sort the data, or maybe select all the column/row.
The sorting (or any other event handling) should probably stay external to keep things simple and allow maximum customization, I mentioned it as an example/motivation.

[PaulWaldo]

I have created an implementation that is helping me to achieve this, please see
https://github.com/PaulWaldo/fyne-headertable.
It's a bit rough around the edges, but it may help. If you have any suggestions to better it, please let me know.

[cristim]

@PaulWaldo I wanted to use your little library, seemed neat, but then I realized it's distributed under GPL so it can't be used for non-GPL software. Too bad.

[andydotxyz]

Thanks to support by https://mindpointgroup.com I will be implementing table headers ASAP :)

[andydotxyz]

Headers are now in, and can be tapped if you insert Buttons using CreateHeader and UpdateHeader callbacks which will replace the default